Stopper rod

ABSTRACT

The present invention concerns a stopper rod whose gas tightness and rigidity are improved. In particular, the stopper rod of the present. invention has means for maintaining the compression of the sealing gasket ( 11, 11 ′) in contact with the annular sealing surface ( 10 ) of the body of refractory material when the stopper rod is brought to a high temperature. According to one embodiment of the invention, these means are furnished by a sleeve ( 12 ) comprised of a material with a high coefficient of thermal expansion, the dilatation of which maintains the sealing gasket under compression when the stopper rod is brought to a high temperature.

The present invention concerns a new stopper rod for regulating the flowof a molten metal from a pouring group, for example, steel or cast iron,from a distributor or a casting ladle, and more particularly a one-piecestopper rod that has means for attachment to a lifting mechanism. In aparticular embodiment, the stopper rod has also means for introducing aninert gas, such as argon, into the molten metal bath during continuouscasting operations.

Such a stopper rod and its use are well known to the skilled artisan, inparticular by U.S. Pat. Nos. 4,946,083 and 5,024,422 to which referenceis made. Among others, these documents describe a one-piece stopper rodthat can be attached to a lifting mechanism, comprised of:

a) an elongated body of refractory material that has a boreholepositioned coaxialy with respect to the body of the stopper rod andadapted to receive fixedly a metal rod for its attachment to a liftingmechanism. The axial borehole of the body of refractory material has anenlarged part with an annular sealing surface spaced away from the upperend of the body of refractory material. The means for attachment of themetal rod are generally positioned between the enlarged part and thelower end of the body of refractory material. At its lower end, the bodyof refractory material may have means for introducing gas into themolten metal bath; and

b) an elongated metal rod attached to the body of refractory materialand having an axial borehole communicating in its lower part with theborehole of the body of refractory material. The rod has a collarcarrying an annular sealing surface facing the annular sealing surfaceof the borehole of the body of refractory material to create a gas tightseal. The upper end of the rod is adapted to be attached to a liftingmechanism that permits displacing the stopper rod vertically inside of apouring group such as a distributor.

The stopper rod is connected to a gas supply line, generally but notobligatorily, through the upper end of the rod. Means of attachment tothe body of refractory material are generally positioned between thecollar and the lower end of the metal rod. During the use of such astopper rod, the gas introduced into the stopper rod is conveyed towardthe axial borehole of the body of refractory material in its lower part.Thanks to the means for introducing gas into the molten metal bath thatthe body of refractory material has in its lower part, the stopper rodpermits the introduction of gas into the molten metal bath. The facingannular sealing surfaces of the rod and the body of refractory materialprevent significant losses of inert gas as well as the infiltration ofair.

To improve this tightness even more, it was proposed to place a gastight annular gasket between these sealing surfaces. The U.S. Pat. No.4,946,083, for example, indicates that when a gasket with a thickness ofca. 0.4 mm and of material resistant to high temperatures, e.g.,graphite, is in place, the interface between the annular sea surfaces ofthe rod and the body of refractory material furnishes tightness capableof resisting a pressure up to 3 bars.

The German patent DE-C1-4040189 discloses a stopper comprising anelongated refractory body having

a) an axial borehole and means (locking pin) for attaching a metal rod,

b) a metal rod and

c) a sleeve like body compressing a sealing gasket under actuation of anut.

With the device of this document, it is necessary to correct or fly tunethe tightness when the mounting rod is fixed in the stopper body byscrewing the nut against the upper end of the stopper. The system is notself-tightening and requires human intervention (screwing the nut) whenthe stopper has reached the final temperature. Such an interventionabove the molten metal bath is uneasy and extremely dangerous. The sealis essential for casting high-grade molten metal. In the first place, itis necessary to assure a good protection against the filtration of airresponsible for oxidizing the molten metal during pouring. On the otherhand, it is also indispensable to minimize the losses of inert gas (incase an inert gas is injected through the stopper rod) that causeproduction cost overruns that are far from negligible. Beside thistightness issue, it is also essential that the attachment of the stopperrod to the mechanism remains as rig as possible.

The system in use at the present time still do not however furnish acompletely satisfactory solution for these two points of view.

In conducting this research in this domain, the applicant discoveredthat these problems are due to the fact that Sealing gasket between theannular sealing surfaces of the rod and the body of refractory materialfacing each other does not remain compressed during the entire pouringoperation.

It is believed that this loss of compression in the sealing gasket isdue, at least partially, to the difference between the coefficients ofthermal expansion of the different materials constituting the stopperrod. In particular, under the effect of the temperature to which thestopper rod is raised during pouring, the metal rod becomes highlydilated relative to the body of refractory material. This moresubstantial expansion of the metal rod has the effect of separating theannular sealing surfaces of the rod and the body of refractory materialand consequently reducing the compression of the sealing gasket with allthe adverse consequences involved.

According to the prevent invention, this problem is remedied by givingthe stopper rod particular means for maintaining the compression of thesealing gasket in contact with the annular sealing sure of the body ofrefractory material when the stopper rod is brought to a hightemperature. The stopper rod involved in the present patent applicationis, notwithstanding the means for maintaining the compression of thesealing gasket, similar to that described in U.S. Pat. Nos. 4,946,083and 5,024,422 to which reference is made.

According to the invention, the means of maintaining the compression ofthe sealing surface when the stopper rod is brought to high temperatureis furnished by a sleeve presenting the form of a cylinder open at itsends, which is fitted on the metal rod. The present invention concerns aone-piece stopper rod that can be attached to a lifting mechanism,comprised of:

an elongated body of refractory material, having

(i) a borehole positioned coaxially with respect to the body and adaptedto receive fixedly a metal rod for its attachment to a liftingmechanism, the axial borehole having an enlarged part that presents anannular sealing surface spaced away from the upper end of the body;

(ii) means for attaching the said metal rod;

an elongated metal rod fastened to the body adapted at its upper end tobe attached to a lifting mechanism for vertically displacing the stopperrod inside of a pouring group; and

a sleeve having a sealing surface at its lower end facing the sealingsurface of the body, the stopper rod having means located on the metalrod for blocking the sleeve, the stopper being characterized in that thesleeve is formed of a material having a coefficient of thermal expansiongreater than that of the metal rod and having a sufficient length sothat, under the effect of the temperature to which the stopper rod isbrought during pouring, the sleeve expand sufficiently to at leastcompensate for the effect of expansion of the metal rod.

According to a particular embodiment of the invention, the stopper rodcan be connected to a gas supply line. Therefore, said elongated body ofrefractory material has, at its lower end, means for introducing gasinto the molten metal bath and said metal rod has an axial boreholecommunicating in its lower part with the borehole of the body ofrefractory material.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary view in cross section of the upper end of astopper rod according to a particular mode of implementing theinvention.

On this FIGURE, the stopper rod 1 is comprised of an elongated body ofrefractory material 2 with an axial borehole 3 extending from its upperend 4 toward its lower end (not shown). At its lower end, the body ofrefractory material may or may not be provided with means forintroducing inert gas (not shown) into the metal bath. The body ofrefractory material also has means 5 for attachment of a metal rod 6.The metal rod 6 also may have an axial borehole 7 that runs through itfrom its upper end 8 toward its lower end 9. The upper end 8 may bedesigned to receive a connector (not shown) for supplying an inert gas.Furthermore, the upper end 8 of the rod is adapted to be fixed to alifting mechanism (not shown). A gas under pressure, such as argon, maybe introduced into the axial borehole 3 of the body of refractorymaterial by means of the rod 6 and is conveyed to the metal bath throughthe lower end of the body of refractory material.

The body of refractory material 2 has an enlarged part 10 that forms asealing surface. Two graphite gaskets (11 and 11′) rest on this sealingsurface and thus prevent the infiltration of air or losses of inert gas.A sleeve 12 is fitted on the rod 6 and maintains the gaskets 11 and 11′under compression. The upper part 13 of the sleeve is blocked by awasher 14, itself retained by a nut 15.

Preferably, the washer 14 is in contact with the upper end 4 of the bodyof refractory material 2 in order to give the assembly an increasedrigidity.

The sleeve 12 is comprised of a material having a coefficient of thermalexpansion greater than that of the metal rod 6 and has a sufficientlength so that under the effect of the temperature to which the stopperrod is brought during pouring it expands sufficiently toward the lowerend of the metal rod to at least compensate for the effect of theexpansion of the metal rod.

Preferably, the expansion of the sleeve compensates essentiallyprecisely with the expansion of the metal rod.

As can be seen in FIG. 1, the sleeve 12 can project at the upper end 4of the body of refractory material 2 if this is necessary to permit asufficient length of the sleeve. In this case, a washer 14 provided witha shoulder 16 that permits blocking the sleeve 12 while assuring acontact between the washer 14 and the upper end 4 of the body ofrefractory material 2 is preferred.

The sleeve 12 is fitted on the metal rod 6 and forms with it a freeassembly, turning, sliding or just sliding. The upper end 13 of thesleeve 12 just butts on the blocking means 14 and 15 located fixedly onthe metal rod 6 so that, under the effect of expansion, the sleeve 12 iselongated anally only in the direction opposite the said blocking means.

According to one embodiment of the invention, the blocking means arecomprised of a collar similar to that described in the U.S. Pat. Nos.4,946,083 and 5,024,422, to which reference is made.

The material constituting the sleeve as well as its length are chosen asa function of the dimensions and the materials constituting the metalrod (generally machined from a bar of steel with a coefficient ofthermal expansion of the order of 12.5 μm° C⁻¹) and the body ofrefractory material (typically comprised of a refractory materialobtained by isostatic pressing with a coefficient of thermal expansionof 3-6 μm° C⁻¹).

The material constituting the sleeve as well as its length are easilydetermined from the basic principles of thermal physics.

Starting with the values thus determined in a first approximation andwhich generally furnish excellent results, it is then possible tooptimize the system by trial an error without any difficulty.

According to the invention, the sleeve is comprised of a material with ahigh coefficient of thermal expansion capable of resisting the elevatedtemperatures to which the stopper rod is subjected during pouring. Forexample, refractory materials with a high coefficient of thermalexpansion such as fritted magnesia can be used. The preferred materialsfor this application are found among metals or metal alloys with a highcoefficient of thermal expansion and having a high melting point.

In general, a material having a coefficient of thermal expansion between1.1 and 3 times that of the material of the steel rod is chosen for thesleeve.

Stainless steel (e.g., having a coefficient of thermal expansion of theorder of 17.5 μm° C⁻) is particularly appropriate when the metal rod ismachined from a steel bar having a coefficient of thermal expansion ofthe order of 12 μm° C⁻.

The body of refractory material is typically formed of a conventionalrefractory material such as a refractory material based on aluminasilica graphite commonly used. A typical composition is, for example, inpercentage by weight, Al₂O₃: 53%, SiO₂: 13%, carbon: 31% and about 3% ofother materials such as zirconia ZrO₂, for example.

An annular gasket 11 that is gas-tight is preferably placed between thesealing surfaces. A gasket of graphite with a thickness between 0.2 and30 mm is typically used.

According to the present invention, one or more conventional gaskets areused. The annular sealing gasket(s) are placed between the lower surfaceof the sleeve and the sealing surface of the body of refractorymaterial.

The best results were observed when two graphite gaskets of 9 mm eachwere placed between the lower surface of the sleeve and the sealingsurface of the body of refractory material.

According to a preferred form of implementing the invention, the sealingsurface formed by the enlarged part 10 of the body of refractorymaterial 2 as well as the sealing gasket(s) 11 (and 11′) are flat. Itwas in fact found that in this case a much better preservation of thecompression of the gasket(s) 11 (and 11′) was obtained. In oneimplementation example, a body of refractory material obtained byisostatic pressing (coefficient of thermal expansion: 3.6 μm° C⁻) eachhaving a thickness of 9 mm. The metal rod is machined from a steel barwith a coefficient of thermal expansion of 12.5 μm° C⁻¹.

If a sleeve of stainless steel with a coefficient of thermal expansionof 17.5 μm° C⁻¹ is used, it is calculated that the sleeve should have alength of ca. 61 mm.

According to another embodiment of the invention, the means formaintaining the compression of the sealing gasket when the stopper rodis brought to a high temperature are furnished by positioning thefixation point of the metal rod to the body of refractory material at apoint situated between the sealing surface and the upper part of therod. In this case, more important is the expansion, more the sealinggasket is compressed by the metal rod that expands under the effect ofthe temperature to which the stopper rod is brought during pouring.

According to a particular form of implementing the invention, thestopper rod also has means for preventing the metal rod from separatingfrom the body of refractory material. Thus, if a metal insert having athreaded axial internal borehole anchored in the body of refractorymaterial was used as the means of fixation of the rod to the body ofrefractory material, the rod will be prevented from becoming unscrewedfrom the insert by furnishing it with a pair of parallel plane surfacesat the point of emergence from the body of refractory material and bysupporting on these plane surfaces an integral forked flange joinedfixedly to the body of refractory material. This fixed joint can berealized by a pin inserted in a shaft effected through the forked flangeand extending into the body of refractory material. In this case, thewasher according to the present invention can advantageously also playthe role of the forked flange.

According to another particular embodiment of the invention, the body ofrefractory material of the stopper rod is comprised at least partiallyof a refractory material relatively impermeable to gases. Quiteparticularly, the body of refractory material is comprised of at leasttwo different refractory materials, a first part comprised of a mixturerelatively impermeable to the gases substantially surrounding the regionin which the sealing gasket is positioned and a second part comprised ofa refractory material resistant to corrosion by molten metals.

What is claimed is:
 1. A stopper rod for use in the casting of moltenmetal comprising: a) an elongated body of refractory material having alower end, an upper end, a longitudinal axis extending between the lowerand upper ends, an inner surface open to the upper end and defining aborehole along the longitudinal axis, the borehole having an enlargedportion comprising an annular sealing surface spaced away from upperend, and a fastener within the borehole spaced further from the upperend than the annular sealing surface; b) a metal rod comprising a firstmaterial having a first coefficient of thermal expansion, a first end,and a second end, the first end fixedly secured to the fastener, and thesecond end adapted to be attached to a lifting mechanism capable ofvertically displacing the stopper rod inside of a casting vessel; c) ablocking device attached to the metal rod and contacting the upper endof the body; and d) a sleeve comprising a second material having asecond coefficient of thermal expansion greater than the firstcoefficient of thermal expansion, the sleeve having a second sealingsurface cooperating with the annular sealing surface, the sleeve securedagainst the annular sealing surface by the blocking device, and having alength at least sufficient to compensate for thermal expansion of themetal rod during casting.
 2. The stopper rod according to claim 1,wherein the stopper comprises at least one annular sealing gasket incontact with the annular sealing surface.
 3. The stopper rod accordingto claim 2, wherein the annular sealing gasket and the annular sealingsurface are flat.
 4. The stopper rod according to claim 1, wherein themetal rod has an inner surface defining a throughbore between the firstand second ends, and the throughbore at the first end fluidlycommunicates with the borehole of the body.
 5. The stopper rod accordingto claim 1, wherein the lower end of the body comprises a gas injectorfor introducing gas into the molten metal.
 6. The stopper rod accordingto claim 1, wherein the sleeve projects beyond the upper end of thebody.
 7. The stopper rod according to claim 1, wherein the secondcoefficient of thermal expansion is from 1.1 and 3 times that of thefirst coefficient of thermal expansion.
 8. The stopper rod according toclaim 1, wherein the second material comprises stainless steel, thefirst material comprises steel, and the body is formed by isostaticpressing.
 9. The stopper rod according to claim 1, wherein the blockingdevice comprises a collar.
 10. The stopper rod according to claim 1,wherein the blocking device comprises a nut threaded on the metal rod.11. The stopper rod according to claim 1, wherein the blocking devicecomprises at least one washer.
 12. The stopper rod according to claim11, wherein the washer comprises an internal shoulder so that the sleevecan project beyond the upper end of the body while the washer contactsthe upper end of the body.
 13. The stopper rod according to claim 12,wherein the stopper comprises a secondary fastener that prevents themetal rod from separating from the body.
 14. A stopper rod for use inthe casting of molten metal comprising: a) an elongated body ofrefractory material having a lower end, an upper end, a longitudinalaxis extending between the lower and upper ends, an inner surface opento the upper end and defining a borehole along the longitudinal axis,the borehole having an enlarged portion comprising an annular sealingsurface spaced away from upper end, and a fastener within the boreholespaced further from the upper end than the annular sealing surface; b) ametal rod comprising a first material having a first coefficient ofthermal expansion, a first end, and a second end, the first end fixedlysecured to the fastener, and the second end adapted to be attached to alifting mechanism capable of vertically displacing the stopper rodinside of a casting vessel; c) a blocking device comprising a nut and awasher, the nut being fixedly secured to the metal rod and securing thewasher against upper surface of the body; d) at least one annularsealing gasket contacting the annular sealing surface; and e) a sleevecomprising a second material having a second coefficient of thermalexpansion greater than the first coefficient of thermal expansion, thesleeve having a second sealing surface cooperating with the annularsealing gasket, the sleeve secured against the annular sealing gasket bythe blocking device, and having a length at least sufficient tocompensate for thermal expansion of the metal rod during casting. 15.The stopper rod according to claim 14, wherein the washer comprises aninternal shoulder so that the sleeve can project beyond the upper end ofthe body.